Circuit interruption breakers are known to form arcs during the opening of the breaker contacts. It is also known to provide a plurality of arcing plates to dissipate and cool the arc in a relatively rapid period of time. Means are disclosed in U.S. Pat. Nos. 2,088,489; 2,188,806 and 3,059,081, for generating a gaseous material for enhanced cooling and physically motivating the arc to within the high voltage region of the arc chute assembly.
It is also known, in certain rated breakers, to employ a magnetic field to magnetically direct the arc to within the arc chutes. U.S. Pat. Nos. 2,442,199 and 2,610,937 disclose the use of slotted plate arc extinguishers for enhancing the transfer of the arc to within the plates. U.S. Pat. No. 2,639,357 discloses the use of parallel resistance across a pair of arcing contacts to reduce the current through a current limiter as well as to assist in extinguishing the arc. Examples of circuit breakers employing vented casings can be found in U.S. Pat. Nos. 3,171,936; 3,506,799; and 4,260,863. The venting is described mainly for preventing damage to the breaker casings. U.S. Pat. Nos. 3,515,829 and 3,516,090 disclose a current limiting breaker having venting means out one end of the breaker housing, through a vented baffle.
One example of an efficient current limiting circuit breaker can be found in U.S. Pat. No. 4,375,021 entitled "Rapid Electric Arc Extinguishing Assembly For Circuit Breaking Devices Such As Electric Breakers" which patent is incorporated herein for purposes of reference.
U.S. Pat. No. 4,019,006 to Manfred Strossner, discloses double chamber venting wherein an arc quenching chamber is situated within an outer chamber, and the arc gases pass first through the arc quenching chamber before venting to the atmosphere via the outer chamber. This patent teaches that the back pressure and noise generated by the arc gases can be reduced by the double chamber design.
In current limiting and circuit interrupting devices rated at 400 amperes current, the factor of merit for energy dissipation is described in the terms of the I.sup.2 t value which represents the product of the square of the current at interruption and the interruption time. The larger the I.sup.2 t value, the lower the efficiency of the breaker for dissipating the energy created during the arc occurrence. For circuit breakers of the type currently manufactured and having a 400 ampere rating, the I.sup.2 t value can be greater than 3.times.10.sup.6 ; that is, in excess of 3 million.
Attempts to further decrease the I.sup.2 t value at larger interrupting currents have not been entirely successful because of the large amount of noise that is generated by the expulsion of the arc gases out through the vents in the breaker housing. Noise levels of as high as 137 decibels can be created when interrupting currents of tens of thousands of amperes. This level of noise requires extensive accoustical mufflers and baffles to decrease the noise level to acceptable levels. Another device employed to reduce the noise is a parallel resistor of the type described earlier, which is fairly effective. The expense involved with the use of a resistor does not always cost-justify its use in current limiting breakers to date.
The purpose of this invention therefore, is to provide an improved current limiting device having an I.sup.2 t value substantially lower than present state of the art devices with acceptable noise levels at interruption, without the requirement of resistors or baffles.